Table 4-1	Identified chemical species in the organic phase in TBP-dodecane-HNO3 (aq) system

Fig. 4-3 Concentration of the species present in the organic phase as a function of nitric acid concentration in aqueous solution, TBP concentration being 1 mol/l

Successful reprocessing of spent nuclear fuels requires the chemical separation of uranium (U) and plutonium (Pu) for recovery. Spent fuel is dissolved in a concentrated nitric acid and then U and Pu are extracted from the aqueous nitric acid solution into an organic (specifically, dodecane) phase using TBP (tributyl phosphate). However, since TBP is basic in chemical nature, it extracts nitric acid (HNO3) and as well as metal ions. In particular, it is known that interaction between TBP and HNO3 leads to formation of a variety of complexes. Thus, to understand the behavior of U and Pu in solution, knowledge of the behavior of HNO3 is also necessary. The TBP-HNO3 extraction system has been studied for more than 40 years, yet there is still no definitive agreement on such fundamental questions as what complexes are actually produced. Based on a detailed equilibrium theory which takes activity changes into account, we have obtained definitive results on the issue of complex formation in reactions between TBP and HNO3, and also on hydration of TBP in a TBP-nitric acid solution. Table 4-1 shows chemical species identified in the dodecane phase. Fig. 4-3 shows how the concentrations of species present in the organic phase change with the nitric acid concentration, while the concentration of TBP remains at 1 mol/l. The results obtained in this study are expected to be useful in conducting fuel reprocessing more safely and effectively.

Reference H. Naganawa et al., Complex Formation between Tributyl Phosphate and Nitric Acid and the Hydration of the Complexes in Dodecane, Bull. Chem. Soc. Jpn., 70 (4), 809 (1997).

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